Giant impacts on exoplanets might be detectable from Earth, providing another means to study these worlds. (credit: Don Davis/NASA)

Can we detect asteroid impacts with rocky extrasolar planets?

by Michael Paine
Monday, December 11, 2006

Caution: This article contains speculation, ballpark estimates, mathematics, and tables!

Sixty-five million years ago a chunk of rock and ice perhaps 15 kilometers across collided with the Earth and wiped out many creatures, including the dinosaurs. This impact, known as the Chicxulub impact, must have created a spectacular flash. Was it bright enough to be detected as far away as Sirius? How many impacts like this are occurring in our region of the Milky Way? Have we any chance of detecting these impacts? These questions can be answered, in part, with a little mathematics, a few facts from astronomy and the audacity to assume that our solar system is “typical”.

In 1994 astronomer Alan Stern asked a similar question and evaluated the detection of newly formed planets that were still glowing red hot from the accretion process. He also calculated the detectability of giant impacts with planets like Neptune. Building on this work, let us consider the effects of a Chicxulub-size impact with planets similar to the Earth.

Impact frequency

The Earth suffers a Chicxulub-size impact every 100 million years or so. There are four rocky planets in our solar system, so we can expect one to be hit every 25 million years. However, half of these impacts will be on the side facing away from the Earth so a visible impact occurs, on average, once every 50 million years. During the first billion years of our solar system, though, the impact rate was at least 1,000 times as intensive as the present rate. This means the average rate over the four-billion-year life of the solar system is some 250 times the current rate, or one impact every 200,000 years.

Number of stars in our region of the solar system

It is about four light-years to the nearest star beyond our Sun, but that is fairly close compared with most stars. On average there is about one star for every 500 cubic light-years of space. This does not seem like much, but distance raised to the third power quickly produces large numbers:

So if we could detect a Chicxulub-size impact from 1,000 light-years away then it should only take a few weeks of observations to find one.

The impact fireball

The impact of an asteroid (or comet) into the surface of a rocky planet typically generates a hemispherical fireball with a diameter some 14 times the diameter of the asteroid. This means the Chicxulub fireball was about 200 kilometers in diameter (and, as it happens, the Chicxulub crater has a similar diameter.)

When fully expanded, the temperature of the gases in the fireball are about twice the surface temperature of our Sun, or about 12,000 kelvins. It is also convenient to compare the luminosity of the fireball with that of the Sun. This, in turn, can be used to compare brightness at a distance. The relative luminosity, or ratio of the fireball luminosity to the Sun’s luminosity, is proportional to the square of the diameter ratio and the fourth power of the temperature ratio:

Relative luminosity = (Fireball diameter/Sun diameter)² x (Fireball temperature/Sun temperature)⁴

The Chicxulub fireball works out as having a relative luminosity of 3.6x10-7 (that is one divided by about three million) so the task of detecting the event a long way from our solar system seems daunting. However, that is not the end of the bad news. The fireball tends to mask itself so that not all of the light is emitted. This emissivity can drop to one percent or less depending on the characteristics of the impact.

The good news is that Venus has a luminosity that is about one billionth of that of the Sun, so if we can detect a Venus-like planet around another star then it is likely that we could detect a Chicxulub impact, provided the detection system was fast enough to record a fireball that lasts about a minute. There might also be other, more effective ways to detect the complex radiation from these impacts but this analysis is confined to visible light.

Detection from a distance

The apparent brightness of stars is measured as “magnitude”. Magnitude is a type of reverse logarithmic scale. One interval of magnitude corresponds with a brightness decrease of a factor of 2.5. The faintest naked-eye star has a magnitude of about 6.5. Sirius has a magnitude of –1.4. For the purpose of comparing the brightness of stars an absolute magnitude is used and is based on the brightness of the star if it was viewed from about 32 light-years away (10 “parsecs”).

We now have enough information to take a guess at the apparent magnitude of a Chicxulub-size impact at various distances:

Apparent Magnitude of Chicxulub Impact for Various Emissivities

Typical Spaceguard telescope systems (looking for near-Earth asteroids) have a limiting magnitude (detection threshold) of about 21 for a 100-second exposure. Therefore a Chicxulub-size impact viewed from 10 light-years away would be barely detectable (20.9), even at 100% emissivity. It seems that the Spaceguard system has no chance of detecting an extrasolar impact.

Telescopes are currently being developed that will look at large areas of the sky for transient astrophysical events. For example, NASA’s Swift spacecraft recently detected a giant stellar flare some 135 light-years away. These new telescopes should have greater sensitivity than the Spaceguard telescopes (many of which are hand-me-downs). It is therefore possible that one of these new systems will eventually come across a major extrasolar impact.

So what: why bother with extra-solar impacts?

First of all, astronomers looking for other types of transient events should be aware that they may, serendipitously come across an impact event. They should understand the likely signatures of impact events.

The detection of extrasolar impacts would give scientist a better idea of whether our solar system was typical. In particular it would be an indicator of the presence of other rocky planets. The characteristics of the impact flash might also reveal the chemistry of the surface of the planet, just like the Deep Impact mission that collided with comet Tempel 1 in July 2005.

Finally, the detection of these events would give a better understanding of the role that impacts play in the evolution of planetary surfaces and, perhaps, the evolution of life on those planets.

Bibliography and acknowledgements

Bondi, H, 1970, “Astronomy of the Future”, Quarterly Journal of the Royal Astronomical Society, Vol. 11, p.443 (astronomy of the day was missing transient events)

Collins G, Melosh H and Marcus R, 2005, “Earth Impact Effects Program: A Web-based computer program for calculating the regional environmental consequences of a meteoroid impact on Earth”, Meteoritics & Planetary Science Vol.40 No. 6, pp817–840.

Harris A, 1998, “Evaluation of ground-based optical surveys for near-Earth asteroids”, Planet. Space Sci. Vol. 46 No. 2/3, pp283-290.

Lewis J, 2000, Comet and asteroid impact hazards on a populated Earth, Academic Press.

Stern A, 1004, “The detectability of extrasolar terrestrial and giant planets during their luminous final accretion”, The Astronomical Journal, Vol. 108 No. 6, pp2312-2317.

Thanks to Duncan Steel, Paul Davies, John Lewis, Alan Stern and Jay Melosh for advice on this topic.

Michael Paine (mpaine@tpgi.com.au) is a consulting mechanical engineer based in Sydney. He maintains the web pages of the Planetary Society Australian Volunteers and has been lobbying the Australian government about the Spaceguard issue since funding was stopped in 1996.

Home